# std::ranges::partition_point

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexpr I partition_point( I first, S last, Pred pred, Proj proj = {} ); | (1) | (since C++20)
template< ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate< std::projected<ranges::iterator_t<R>, Proj>> Pred > constexpr ranges::borrowed_iterator_t<R> partition_point( R&& r, Pred pred, Proj proj = {} ); | (2) | (since C++20)

Examines the partitioned (as if by ranges::partition) range `[`first`,` last`)` or r and locates the end of the first partition, that is, the projected element that does not satisfy pred or last if all projected elements satisfy pred.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | iterator-sentinel defining the partially-ordered range to examine
r | - | the partially-ordered range to examine
pred | - | predicate to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

The iterator past the end of the first partition within `[`first`,` last`)` or the iterator equal to last if all projected elements satisfy pred.

### Complexity

Given N = ranges::distance(first, last), performs O(log N) applications of the predicate pred and projection proj.

However, if sentinels don't model std::sized_sentinel_for<I>, the number of iterator increments is O(N).

### Notes

This algorithm is a more general form of `ranges::lower_bound`, which can be expressed in terms of `ranges::partition_point` with the predicate [&](auto const& e) { return std::invoke(pred, e, value); });.

### Example

Run this code

    #include <algorithm>
    #include <array>
    #include <iostream>
    #include <iterator>

    auto print_seq = [](auto rem, auto first, auto last)
    {
        for (std::cout << rem; first != last; std::cout << *first++ << ' ') {}
        std::cout << '\n';
    };

    int main()
    {
        std::array v {1, 2, 3, 4, 5, 6, 7, 8, 9};

        auto is_even = [](int i) { return i % 2 == 0; };

        std::ranges::partition(v, is_even);
        print_seq("After partitioning, v: ", v.cbegin(), v.cend());

        const auto pp = std::ranges::partition_point(v, is_even);
        const auto i = std::ranges::distance(v.cbegin(), pp);
        std::cout << "Partition point is at " << i << "; v[" << i << "] = " << *pp << '\n';

        print_seq("First partition (all even elements): ", v.cbegin(), pp);
        print_seq("Second partition (all odd elements): ", pp, v.cend());
    }

Possible output:

    After partitioning, v: 2 4 6 8 5 3 7 1 9
    Partition point is at 4; v[4] = 5
    First partition (all even elements): 2 4 6 8
    Second partition (all odd elements): 5 3 7 1 9

### See also

ranges::is_sorted (C++20) | checks whether a range is sorted into ascending order (algorithm function object)
ranges::lower_bound (C++20) | returns an iterator to the first element not less than the given value (algorithm function object)
partition_point (C++11) | locates the partition point of a partitioned range (function template)

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